U.S. patent number 4,840,178 [Application Number 07/050,940] was granted by the patent office on 1989-06-20 for magnet for installation in the middle ear.
This patent grant is currently assigned to Richards Metal Company. Invention is credited to Dennis Bojrab, Timothy D. Gooch, Jorgen Heide, Anthony D. Prescott.
United States Patent |
4,840,178 |
Heide , et al. |
June 20, 1989 |
Magnet for installation in the middle ear
Abstract
A magnet assembly for location around portions of bones in the
middle ear where one portion of the assembly is a magnet and
another portion of the assembly is a magnetic material, the
portions being hinged to one another so that they are held around
the bone by the magnetic field and being adapted for optimal
coupling with a magnetic field produced by a coil in a magnetic
induction hearing aid.
Inventors: |
Heide; Jorgen (Cordoba, TN),
Gooch; Timothy D. (Memphis, TN), Prescott; Anthony D.
(Arlington, TN), Bojrab; Dennis (Bloomfield Hills, MI) |
Assignee: |
Richards Metal Company
(Memphis, TN)
|
Family
ID: |
21968459 |
Appl.
No.: |
07/050,940 |
Filed: |
May 15, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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837708 |
Mar 7, 1986 |
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Current U.S.
Class: |
600/25; 335/219;
335/231; 335/275; 335/279; 381/312; 381/326 |
Current CPC
Class: |
H04R
25/606 (20130101) |
Current International
Class: |
H04R
25/00 (20060101); H04R 025/00 () |
Field of
Search: |
;128/419R,420.5,420.6
;381/68.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lastova; Maryann
Attorney, Agent or Firm: Pravel, Gambrell, Hewitt, Kimball
& Krieger
Parent Case Text
This application is a continuation-in-part of copending
application, Ser. No. 837,708, filed Mar. 7, 1986.
Claims
We claim:
1. A magnet assembly for coupling with the magnetic coil of a
hearing said, comprising:
a first separate section sized and shaped to partially surround at
least a portion of a bone in the ossicular chain;
a second separate section sized and shaped to cooperate with the
first section and clamp onto said portion of a bone in the
ossicular chain,
at least one of the first and second sections comprising a magnet
and the other section formed of material that is magnetically
attracted to the magnet.
2. The magnet assembly of claim 1, further comprising:
hinge means for connecting said first and second sections, said
hinge means contacting said first and second sections so that a
portion of each section is on both sides of the pivot point of said
hinge means;
the sections being shaped and dimensioned so that force applied to
outer surfaces of each section in the direction of the other
section on one side of said pivot point will cause the portion of
each section on the other side of said pivot point to move apart,
removal of the force allowing said section portions to move
together and clamp onto a portion of a bone in the ossicular chain
because of the magnetic attraction of said sections.
3. The magnet assembly of claim 2, wherein one section is formed of
a magnet and the other section of magnetically attracted
material.
4. The magnet assembly of claim 2, wherein both sections are
magnets with poles located so that they attract each other.
5. The magnet assembly of claim 2, wherein both sections are coated
with a biocompatible polymer.
6. The magnet assembly of claim 2, wherein said hinge means
includes one section having a fulcrum and the other section having
a fulcrum groove for receiving the fulcrum.
7. The magnet assembly of claim 2, wherein first and second pieces
include opposing faces and hollowed-out portions designed to engage
the incus.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to hearing aids and, more
particularly, to a hearing aid using magnetic induction to
reproduce sound.
2. Description of the Prior Art
Hearing aids are useful in restoring lost aural perception to those
persons having mild to severe loss of hearing. Conventional hearing
aids have a microphone, amplifier circuitry, a battery and a
speaker. The microphone receives the sound energy and transforms
the sound energy into an electrical signal which is then amplified
and filtered. This amplified signal is transformed back to acoustic
energy by the speaker and transmitted to the person's middle ear
for perception of the sound. These hearing aids can be placed
behind the ear, with only the receiver being placed inside the ear
canal. Alternatively, in-the-ear hearing aids are available which
are placed in the outer ear and have portions extending into the
ear canal.
There are a number of problems with conventional hearing aids. All
conventional hearing aids are visible to some extent and therefore
have an undesirable cosmetic appearance. Conventional hearing aids
have acoustic feedback problems because sound energy can escape
from the ear canal and be detected by the microphone, generating a
feedback-related whistle. Additionally, sound reproduction is often
lacking in clarity because of distortions generated by standing
waves existing in the closed cavity between the hearing aid and the
tympanic membrane and poor mechanical reproduction by the
speaker.
It has been suggested that a magnetic induction hearing aid would
remove many of these problems. A magnet or other item having a
magnetic field is placed in the middle ear, either in contact with
the tympanic membrane or in contact with other portions of the
middle ear. Electrical circuitry and a coil would generate a
magnetic field having the same frequency as the external sound. The
magnetic field generated by the coil would interact with the field
of the magnet and cause the magnet to vibrate at the same frequency
as the magnetic field. The vibration of the magnet would then cause
the attached portion of the middle ear to vibrate, resulting in a
perception of the external sound.
A magnetic induction hearing aid would overcome feedback or
distortion problems of conventional hearing aids because there
would be no significant air movement in the ear canal, resulting in
insufficient energy escaping around the hearing aid to generate a
feedback problem. There would be no standing waves generated to
cause distortion because there are no appreciable sound waves at
all.
Attempts to use magnetic induction hearing aids have been reported.
An early attempt placed a coil in conjunction with a small piece of
iron on the tympanic membrane, which was excited by an external
coil placed over the ear canal. This system did allow the
perception of the stimulus, but had the side effect of producing
discomfort and pain for the wearer. A later attempt glued a small
magnet to the umbo and used an external coil placed over the ear of
the wearer to cause the sympathetic vibrations of the magnet. This
apparatus required approximately 7.9 ma to produce a 0 db hearing
level at 1000 Hz.
In an article entitled Audition via Electromagnetic Induction, Arch
Otolaryngol 23 (July 1973), Goode et al describe a number of tests.
One test attached a magnet to the tympanic membrane and located a
coil in the ear canal 3 mm from the magnet. The coil was driven
externally by an audiometer. This development required only 0.7 ma
to produce a 0 db hearing level at 1000 Hz. Tests were performed
for system fidelity and proved adequate. Another system tested
placed the coil over the ear, drove the coil with an audiometer and
had a magnet glued to portions of the middle ear, but used larger
magnets than in previous tests. One version of this system placed
the magnet on a Silverstein malleus clip which was connected in the
normal manner. Approximately 0.7 ma was required to produce a 0 db
hearing level using these arrangements.
These discussions suggested that the use of electromagnetic
induction to produce a hearing aid is possible, but did not teach a
way to develop a practical system. The majority of tests used coils
placed over the ear or adjacent to the ear. Systems using external
coils are not efficient enough for use in conjunction with the low
power requirements dictated by hearing aid batteries. Although one
test indicated that a coil was placed inside the ear canal, an
external amplifier was used to drive the coil. The tests did not
result in a practical device or suggest how a totally in-the-ear
device could be made.
Further, the magnets described in conjunction with the
above-mentioned tests were either glued to portions of the middle
ear and removed after short periods of time or were connected to
malleus clip and inserted for a longer duration. Neither of these
attempts resulted in a magnet that could be implanted for extended
periods of time with no danger of rejection by the body, have no
movement in relation to the middle ear and yet have as little
weight as possible.
SUMMARY OF THE INVENTION
The present invention is directed to a magnetic induction,
in-the-ear hearing aid where all the elements of the hearing aid
are placed within the ear canal and the middle ear. A microphone,
amplifying electronics, battery and driving coil are placed within
a single housing which is custom molded for each wearer and placed
deep within the ear canal.
The amplifier is one of two types, either Class A or Class B,
depending on volume levels required. The coils are matched to the
particular amplifier type to provide optimal efficiency for a given
design. The coil is formed of a number of turns of wire wound over
a mumetal core, which is used to increase magnetic field strength.
The coil is placed close to the magnet to allow optimal coupling of
the magnet's field with the magnetic field produced by the
coil.
The magnet is formed of a neodymium-iron material allowing a very
high strength magnetic field to be developed by a very small
magnet. Since this material corrodes when placed in an animal body,
it is coated with a biocompatible material.
The magnet is formed of a neodymium-iron material allowing a very
high strength magnetic field to be developed by a very small
magnet. Since this material corrodes when placed in an animal body,
it is coated with a biocompatible material.
The magnet can be installed around the incus or other bone in the
middle ear. The incus is a location which is easily accessible in
individuals having undamaged physical middle ear structures and is
located close to the tympanic membrane so that efficient magnetic
coupling can occur between the magnet and the coil of the hearing
aid.
The magnet is preferably a two piece magnetic structure hinged at
one end so that the structure can be opened for placement around
the incus and then is self closing without need for crimping or
bending of wires or use of other materials. One piece of the
assembly is comprised of a magnet, preferably, samarium cobalt,
while the other piece is comprised of magnetic material. This
magnetic material can be paramagnetic, ferromagnetic or can be an
actual magnet. The magnet is coated with biocompatible materials to
prevent corrosion or other adverse body reactions.
The hinge between the two components can be a separate hinge or can
be a pivoting structure formed by the interaction of the two
pieces. The structure is self closing in that an open gap exists on
the side opposite the hinge after the structure has been placed
around the incus. The existence of the gap causes the pieces that
form the structure to continually try to move together because of
the magnetic attraction that exists in the open gap of the
structure. This allows portions of the bone to diminish in size and
yet have the magnet continue to remain firmly affixed without need
for further attention by a physician. The magnet is adapted for
optimal shape for coupling with the field produced by the magnetic
coil of the hearing aid, thereby increasing the efficiency and
battery life of the hearing aid.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the invention can be obtained when the
detailed exemplary embodiment set forth below is considered in
conjunction with the following drawings, in which:
FIG. 1 is a cross-sectional view of a human ear with a magnetic
induction hearing aid according to the present invention placed in
the ear canal;
FIG. 2 is an electrical schematic diagram of one embodiment of a
circuit utilizing a Class A amplifier designed according to the
present invention;
FIG. 3 is an electrical schematic diagram of a second embodiment of
a circuit utilizing a Class B amplifier designed according to the
present invention;
FIG. 4 is a side view of a malleus clip having a magnet mounted
thereon;
FIGS. 5a, 5b, 5c and 5d are, respectively, cross-sectional top and
side views of a magnets formed according to the present
invention;
FIG. 6 is a partial cross-sectional view of a middle ear showing an
magnet implanted according to the present invention;
FIG. 7 is a cross-sectional view of an eardrum or tympanic membrane
and malleus in which a magnet mounted to a malleus clip is
connected to the malleus;
FIGS. 8a, 8b and 8c are schematic illustrations of coils formed
according to the present invention;
FIG. 9 is a side view of a magnet according to the present
invention;
FIG. 10 is a perspective view of the magnet of FIG. 9; and
FIG. 11 is a partial cross-sectional view of a middle ear showing a
magnet implanted according to the present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
Referring to FIG. 1, the letter H refers generally to a hearing aid
according to the present invention and is shown installed in an ear
canal 34. The hearing aid H has a housing 30 enclosing a microphone
20, an amplifier 22, a volume control 24, a battery 26 and a coil
28. The hearing aid H is located deep in the ear canal 34 so that
the coil 28 is located near a coated magnet 32, with 2.5 mm being a
desirable distance for this separation. This distance is
sufficiently close to reduce the inverse relationship of distance
to magnetic field strength and yet is sufficiently far that the
hearing aid H can be inserted by the wearer with minimal difficulty
and not be in danger of contacting the tympanic membrane 68.
The installation of the hearing aid H deep within the ear canal 34
as shown in FIG. 1 eliminates any negative cosmetic effects of a
hearing aid because the hearing aid H is practically undetectable.
A conventional hearing aid cannot be inserted this deep in the ear
canal 34 because of the standing wave and feedback problems
discussed above. These problems do not occur in a magnetic
induction hearing aid and therefore this deep placement is
possible.
Volume adjustment and battery replacement is accomplished by
removing hearing aid H from the ear canal 34, appropriately
adjusting the volume control 24 or replacing the battery 26 and
reinserting the hearing aid H into the position shown in FIG.
1.
The housing 30 is custom molded to each wearer's ear canal 34. This
is necessary because each wearer has a differently sized and shaped
ear canal. The hearing aid H must be sufficiently close to the
magnet 32 for proper operation and the hearing aid H must be
sufficiently tight within the ear canal 34 to remain in place
during normal use.
A class A amplifier design is shown in FIG. 2. The microphone 20 is
a standard electret microphone as conventionally used in hearing
aids. The amplifier 22c is class A design that is standard in
hearing aid applications. This amplifier is specifically designed
for low voltage operation in conjunction with a single 1.3 volt
battery. The volume control 24 is connected to vary the gain of the
amplifier 22c and thereby change the output signal level applied to
the coil 28a. The coil 28a is designed for use with the class A
amplifier 22c.
Each amplifier used in hearing aids has a recommended output load
impedance which is normally deemed to be the speaker or receiver
impedance. For optimum performance of the hearing aid H, the coil
28a should be designed to match this characteristic desired
impedance across as wide a frequency band as possible. The coil 28a
is a double-ended coil designed to be connected to the battery 26
and to the output of the amplifier 22c. The coil 28a is formed by
winding the appropriate number of turns of wire 72 (FIG. 8a) about
a high permeability core 70. Preferably, the core 70 is comprised
of mumetal to increase the magnetic field strength at the ends of
the coil. The maximum coil size is preferably approximately 9 mm
long and 4 mm in diameter. This size limitation is used in
conjunction with the optimum coil impedance in determining the
number of turns of wire 72 and the gauge of the wire 72 to produce
a coil of the allowed size having the desired impedance.
The class A amplifier 22c is used in situations where the wearer
has only a mild to moderate loss of hearing. The class A design is
used in the mild loss case because the power consumption of the
class A amplifier 22c is lower, but the maximum output is also
lower, necessitating a higher performance or class B design for
high power needs.
Where the wearer has a more severe hearing loss requiring greater
amplification of the sound signal, what is known as a class B
amplifier design as shown in FIG. 3 is used. A class B amplifier
22b is used in the higher volume, higher amplification situations
because it has a power output level higher than that of the class A
amplifier 22c. The trade off for this efficiency is reduced battery
life because of the higher current draw of the class B amplifier
design.
The microphone 20 is connected to a preamplifier stage 22a through
an impedance matching and filter stage 38. The class A preamplifier
22a provides a fixed amount of gain and produces an output signal
which is transmitted to filter capacitors 42 and 44 and the volume
control 24. Appropriately adjusting the volume control 24 changes
the output voltage of the class B output amplifier 22b which in
turn drives coil 28b. As in the class A amplifier 22c, the class B
output amplifier 22b has an optimal load impedance resistance which
is specified by the manufacturer. The coil 28b is designed to have
an impedance which matches this optimal impedance over as broad a
frequency band as is necessary for the given application. The coil
28b is designed with a center tap (FIGS. 8b and 8c) to allow use
with the class B amplifier 22b. An appropriate number of turns of
the appropriate gauge wire 74 are wound around the mumetal core 70
or other high permeability material and connected as required to
the amplifier 22b. The class B amplifier 22b produces greater power
because of its class B design and its push-pull operation, enabling
the coil 28b to produce larger magnetic field densities and thereby
move the magnet 32 a greater distance.
The coil 28 produces a magnetic field varying at the frequency of
the sound waves received by the microphone 20. The coil's magnetic
field then interacts with the magnet 32. A sympathetic vibration of
the magnet 32 occurs at the frequency of the sound waves. This
mechanical vibration of the magnet 32 is then translated into
movement of either the malleus 36 if the magnet 32 is attached to a
malleus clip 60 (FIG. 7) or to vibration of the malleus 36 and the
tympanic membrane 68 if the magnet 32 is inserted between the
malleus 36 and the tympanic membrane 68 as shown in FIG. 6.
It is desirable that the coil 28 be placed in close proximity to
the magnet 32 because a magnetic field decreases with strength
according to the inverse cube law. Therefore, the coil's magnetic
field effecting and interacting with the magnet 32 is radically
diminished as the separation distance increases. This diminishing
interaction directly effects the efficiency of the hearing aid H
and therefore a minimum gap is desirable.
If the magnet 32 is implanted behind the tympanic membrane 68, the
magnet 32 can move by either of two actions. The first movement is
a piston-type action perpendicular to the plane of the membrane 68.
The second action of the magnet 32 is a rocking action about a
horizontal axis of the magnet 32. This rocking does cause the
tympanic membrane 68 and the malleus 36 to vibrate, creating a
sensation of sound. The rocking action is preferable because there
is better magnetic coupling between the magnet 32 and the coil
field, which increases effective acoustic gain and thereby system
efficiency.
The mass of the magnet 32 must be kept at a minimum to further
increase the efficiency of the design so that the coil's magnetic
field does not have to oscillate a large mass and thereof require
additional energy transfer between the coil 28 and the magnet 32.
But the magnet 32 must also be high strength so that the two
interacting magnetic fields, the coil field and the magnet field,
are sufficiently strong to create a sufficient amount of coupling
between the two fields. For this reason it is preferable that the
magnet 32 be formed from the neodymium-iron which has an extremely
high field strength for a given magnet size.
Because the magnet 32 is to be inserted in the human body it is
necessary that the magnet 32 or magnet assembly be biocompatible
and not corrode or cause adverse tissue reaction when placed in the
body. It is also desirable that the magnet become firmly and
permanently attached to the desired portions of the middle ear.
The preferred neodymium-iron magnet, in and of itself, does not
meet these requirements. It corrodes when placed in the body and
therefore is not suitable in its uncoated state for long-term
placement or installation. Therefore, for biocompatibility the
magnet 32 must be coated and sealed with a biocompatible material.
There are two alternative versions of the coated magnet 32, one for
use with the malleous clip 60 and the other for direct implantation
between the tympanic membrane 68 and the malleous 36.
The magnet 32 that is attached to the malleous clip 60 (FIG. 4)
need only be biocompatible to the degree that it does not produce
an infection and does not corrode. For this use, a coating of the
magnet with biocompatible materials such as gold or other
nonresorbable, biocompatible material such as various commonly
available polymers is necessary. No actual mechanical bonding
between the magnet 32 and portions of the middle ear is necessary
because the malleous clip 60 provides the connection with the
malleous 36 and the magnet 32 is firmly mounted on the malleous
clip 60.
For the embodiment of the magnet 32 to be used for direct
implantation between the tympanic membrane 68 and the malleous 36,
different criteria must be considered. It is highly desirable that
this magnet 32 be coated with a bioactive material which will form
a permanent bond with the middle ear. To this end it is preferable
that the magnet 62 (FIGS. 5a, 5b, 5c, 5d) be coated with
hydroxyapatite 64. Hydroxyapatite is a calcium phosphate material
which has a particular crystal structure which resists
biodeterioration and has an outer surface that easily adheres to
tissue that is generated by the adjacent body portion.
Hydroxyapatite is preferred as the material that is useable as an
outer coating material, but other nonresorbable bioactive materials
could be used. Hydroxyapatite is referred to in this specification
because it is the currently preferred material and these references
to hydroxyapatite are intended to include other similar materials.
Coating the magnet 62 with hydroxyapatite 64 and placing the coated
magnet 32 between the tympanic membrane 68 and the malleous 36
results in the magnet 32 becoming part of the middle ear after a
period of time due growth of middle ear tissue and its adherence to
the hydroxyapatite coating 64.
A coating of hydroxyapatite 64 over a bare magnet 62 might possible
by satisfactory if the magnet were sealed from surounding body
fluids. However, because a neodymium-iron magnet is highly
corrodable in an animal body and a complete seal is difficult to
achieve, the magnet 62 first receive a precoating 66 prior to the
final coating of hydroxyapatite 64. This precoating 66 is used to
seal the magnet 62 from the bodily environment and therefore resist
corrosion. The sealant can be a biocompatible material such as gold
or other biocompatible polymers as are used in implantable medical
devices. the precoated magnet is then coated with the
hydroxyapatite 64 or other nonresorbable bioactive materials with
similar properties.
There are several different processes that could be used for
applying the hydroxyapatite coating. The first process is an ion
implantation or sputtering technique wherein the target magnet is
placed inside a vacuum chamber and positioned near a hydroxyapatite
source. The hydroxyapatite source is then bombarded by an electron
beam source from an ion accelerator so that the hydroxyapatite
atoms are stripped from the source material and attracted to the
target material due to electrostatic forces. Alternatively, a
hydroxapatite plasma can be produced by a radio frequency power
source and directed toward the target material. The charged
hydroxyapatite atoms are then driven into the magent 62 or the
precoat 66 by means of an accelerated argon ion beam. This firmly
implants the hydroxyapatite atoms into the magnet 62 or precoat 66
forming a firm bond between the two layers. This process is
continued until a sufficient hydroxyapatite coating thickness is
produced, preferably about one micron.
The ion implantation process in a low temperature process which
allows the magnet 62 retain its magnetism. If the magnet 62 is
subjected to a sufficiently high temperature, it loses its
magnetism and therefore is rendered unusable. For this reason, the
target must be kept at a low temperature which is capable of being
done in the ion implantaion or sputtering technique.
A low temperature process is also important so that the
hydroxyapatite source material retains its preferred hydroxyapatite
structure. If the materials forming the hydroxyapatite are elevated
to a sufficiently high temperature, the hydroxyapatite converts to
tricalciumphosphate which is a bioresorbable material and is not
satisfactory for coating the magnet 62. This is because the
material is resorbed by the body and would eventually disappear
from the magnet 62, leaving the magnet 62 uncoated and not bonded
as desired. Therefore the low temperature ion implantation
technique allows the hydroxyapatite 64 to keep its structure after
being sputtered to the target magnet.
A second process for coating the precoated magnet is a plasma
spraying technique. In this process the hydroxyapatite 64 is in the
form of a powder and is fed through an argon plasma which melts the
hydroxyapatite powder which is then fired onto the surface of the
target magnet. The hydroxyapatite 64 then cools down, solidifies
and is bonded to the precoating material 66. In this process it is
possible to keep the substrate or target material temperature
sufficiently low so as not to demagnetize the magnet 62.
A third process for applying the hydroxyapatite coating material
involves placing the hydroxyapatite material on the surface of the
polymer used as the precoat 66 before the polymer precoating
material is fully solidified. When the biocompatible precoating
polymer material 66 is applied to the magnet 62 in a molten form
there is an interval wherein the precoating material 66 is
sufficiently adhered to the magnet 62 and yet is not completely
solidified. During this tacky or partially fluid state, the
hydroxyapatite material is introduced onto the magnet assembly and
physically pressed into the precoating material 66, therefore
bonding with the precoating material 66 which then completes its
hardening process. In this way, the hydroxyapatite material 64 has
fully interlaced with the precoating polymer 66 which is firmly
attached and sealing the magnet 62. An intermediate biocompatible
coating attached to the underlying precoating material 66 can also
be used to bond the hydroxyapatite 64 to the magnet 62.
Alternatively, the magnet used with the hearing aid H can be
located around the incus 98 (FIG. 11). In this preferred
embodiment, the magnet 100 (FIGS. 9 and 10) is formed of two pieces
which are hinged together. A first piece 102 is generally
cylindrical in shape and has a transverse groove 106 located in the
interior face 105 of the piece 102. The groove 106 is adapted to
contact a portion of the incus 98 when the magnet 100 has been
placed around the incus 98. The first piece 102 also contains a
pivot section or fulcrum 108 which extends from the general body of
the pivot piece 102 and has a rounded cross-section 109 at its end.
A second or mating piece 104 is also generally cylindrical in shape
and also contains a transverse groove 106 adapted for mating with
the incus 98. The second structure 104 contains a second, generally
smaller, transverse groove 110 adapted to cooperate with the pivot
section 108 such that when pivot section 108 is located in the
groove 110, the pieces 102, 104 are hinged at this location through
their magnetic attraction and pivot in relationship to each
other.
Preferably, both of the pieces 102 and 104 are magnets having poles
as shown in FIG. 9. Alternatively, only one of the elements 102 or
104 need be a magnet. The other piece can be a magnetic material,
either paramagnetic or ferromagnetic, such that the material is a
good concentrator of magnetic flux lines. If the magnetic material
is a permanent magnet, the entire magnet assembly 100 has a greater
magnetic field than if one piece is paramagnetic or ferromagnetic,
thereby increasing the efficiency and battery life of the hearing
aid.
The pivot section or fulcrum 108 is further adapted such that the
pieces 102, 104 have a gap 112 when attached to the incus 98. This
gap 112 results in a self-closing feature of the magnet 100 because
the magnet 100 will have a tendency to try to reduce this gap so as
to concentrate the magnetic flux. This self-closing allows the
magnet 100 to remain in contact with the incus 98 even if the incus
98 reduces in diameter as a result of tissue resorption because of
the pressure caused by the attraction between the two pieces 102,
104. Preferably the gap 112 will completely close and the pieces
102, 104 will come into contact before the incus 98 has eroded too
far. When the gap 112 is closed, tissue resorption stops and the
magnet 100 then remains locked onto the incus 98.
The magnet 100 is preferably made of samarium cobalt, but can be
made of neodymium-iron or other magnet materials. In the preferred
embodiment, the pieces 102, 104 have a diameter of 0.090" and a
thickness of 0.030". The incus grooves 106 have a preferable radius
of 0.025", a depth of 0.010" and are 0.040" wide at the piece face.
The pivot section 108 is preferably 0.038" long and the pivot
groove 110 has a radius of 0.007", a width of 0.021" at the piece
face and a depth of 0.008". The magnet 100 is coated with a
biocompatible polymer to prevent corrosion of the magnet, to
prevent rejection of the material by the body and to allow ingrowth
of tissues if desired.
The geometrical proportions and alignment of the magnet 100 are
such that the pole faces are oriented with the magnetic field
produced by the coil 28 in the magnetic hearing aid H to achieve
maximum coupling of the magnetic fields.
The magnet assembly 100 can be adapted to be placed in other
portions of the middle ear other than the incus 98, including the
malleus 36 and the stapes 96, with appropriate design of the pieces
102, 104 for optimal coupling of the magnetic fields given the
magnet 100 location, and for hinge and groove dimensions and
location given the geometry of the portion of the middle ear to
which the magnet 100 is being attached.
Preferably the magnet 100 is designed to be mounted on the incus 98
because this location is the closest, most easily accessible
location relative to the tympanic membrane 68 and allows the use of
basic configurations of the magnet 100 for magnetic field coupling.
The magnet 100 is implanted by grasping the magnet 100 with forceps
114 (see FIG. 9) on the sides 116 of the pieces 102, 104 away from
the incus grooves 106, causing the magnet 100 to pivot open. The
opened magnet 100 is then inserted through an incision formed in
the tympanic membrane 68 until it reaches the incus 98, at which
time the magnet 100 is allowed to close over and clamp onto the
incus 98. The magnet 100 aligns itself with the incus grooves 106
centering on the incus 98. When the magnet 100 closes, it is firmly
attached to the incus 98.
While the pieces 102, 104 that form the magnet 100 are shown in
FIGS. 9-11 as hinged through their magnetic attraction to each
other, alternative embodiments with mechanical hinges using pins or
the like could be used in accordance with the invention. Although
forceps 114 can easily grasp the pieces 102, 104 as shown in FIG.
9, projections or tabs (not shown) could be added at that end of
the pieces 102, 104 to provide more pronounced bearing surfaces for
the forceps.
The foregoing disclosure and description of the invention are
illustrative and explanatory of the invention, and various changes
in the size, shape and materials, as well as in the details of the
illustrated construction and process may be made without departing
from the spirit of the invention, all of which are contemplated as
falling within the scope of the appended claims.
* * * * *